This invention relates to a balanced vane type oil pump, and more particularly a small, light weight and inexpensive oil pump in which a pair of pump cartridges are used as two pumps and the supply of the pressurized oil outputted from each pump is selectively controlled to decrease power consumption.
In a power steering device mounted on a motor car, for example, for decreasing the handle steering power of the driver, a pump for producing pressurized oil for operating the power steering device is usually driven by the engine of the car so that the quantity of the oil discharged from the pump varies in proportion to the number of revolutions of the engine. Accordingly, such a pump is required to have a sufficient capacity to supply a quantity of the pressurized oil necessary to satisfactorily actuate a fluid device even when the engine speed is low, that is, when the discharge quantity of the pump is small.
However, when the pump capacity is increased to satisfy this requirement the discharge quantity of the pump becomes excessive in the high speed range of the engine, thus not only increasing the use of power of the engine for driving the pump but also the fuel consumption the engine.
Accordingly, it has been proposed to divide the pair of pump centridges into two pumps of small capacities and to provide a control member operating to selectively change over the flow passages which supply the pressurized oil from the oil pumps to fluid devices so as to save energy. With this arrangement when the output quantities of the pumps are small the two outputs are combined into a single flow and then supplied to the fluid devices, whereas when the discharge quantities of the pumps increase the output of only one pump is supplied to the fluid devices, while the other pump is connected to the suction side to circulate the oil so as to decrease the power necessary to drive the pumps.
However, the oil pumps just described are constructed to change over the flow passages by taking the number of revolutions of the engine as the reference, so that although it is possible to save the power in the high speed range of the engine, that is at the time when the motor car runs at high speeds, energy loss is inavoidable in the low speed range of the engine. Thus, there are many problems necessary to be solved.
More particulary in the power steering device of the type described above, the quantity of the pressurized oil supplied thereto is important when the power steering device is operating to produce a large power. In other cases, that is when the car is not running or runs without turning, even in the low speed range of the engine the quantity of the pressurized oil may be small. Especially, in a motor car, in many cases it runs according to 10 mode running patterns in a city area so that it is necessary to decrease the power consumption under such low speed running.
This problem can be solved by using a flow passage change over mechanism which operates in response to a load applied to the power steering device. When such change over mechanism is used, even when the engine operates at a high speed so that one pump can supply sufficient quantity of pressurized oil the changing over of the flow passages is performed, thus increasing the power consumption.
It has also been considered to electrically detect the running speed of the motor car to change over the flow passages in response to a detected signal, but since the car speed is not always proportional to the number of revolutions of the engine, that is the discharge quantity of the pump, it is not always possible to decrease the power consumption. Especially, in overloaded trucks or the like, even in a low speed running condition, in many cases, the engine reaches a high speed region thus inducing many problems. In addition, the use of electric detection means and an electromagnetic valve actuated thereby applies a limitation on the construction. The control member for controlling the quantity of the pressurized oil is required to have both performances of selectively changing over the flow passages from both pumps when necessary and of the flow quantity controlling the oil supplied to the fluid pressure devices below a predetermined quantity. Usually, these two performances have been provided by using a pair of spool valves controlling pressurized fluid passages associated therewith. In such a case the pair of spool valves and fluid passages that comprise the control member are incorporated into a pump body together with the pair of pump cartridges, whereby the construction and assembling of the oil pump become complicated.
When using a set of pump cartridges each comprising a rotor with vanes and a cam ring, etc., as two pumps, the following constructional problems arise.
More particularly, since a pair of pump cartridges are utilized as two oil pumps, according to a simplest design, a pair of independent pump chambers formed at symmetrical positions with respect to the rotor axis are communicated with separate discharge passages. One example of this construction is disclosed, for example, in Japanese Preliminary Patent Publication Nos. 49594 and 82868/1980. According to such construction, however, although the constructions of the pump passages and the control member can be simplified, where one pump chamber is connected to the tank side to unload the pump chamber, the other pump chamber performs the pumping action, so that unbalanced load would be applied upon the rotor and its shaft thus degrading the durability and reliability of the movable parts and increasing noise.
U.S. Pat. No. 2,887,060 discloses a balanced type pump free from the problems described above. According to this patent, two independent discharge passages are communicated with a pair of pump chambers symmetrically disposed about the rotor, and paired openings opened in respective pump chambers at symmetrical positions with respect to the rotor axis are combined so as to utilize the pump structure as two independent pumps. This construction, however, increases the number of oil passages so that the connections to respective parts and pipings to the spool valves acting as the control member become complicated.
Since in these pump cartridges, the control member and the oil passages are all contained in a single pump body, the problems described above have a large influence upon the construction, assemblying and the cost of the entire pump.
In the oil pump of the type described above it has been desired to provide a pump having a simple construction, easy to assembly and having small size and light weight. Such desire is especially important for such fluid device as a power steering device installed in a narrow space in an engine room of a motor car.